1. Field of the Invention
The present invention relates to CVD diamond growth. More particularly, the present invention relates to CVD diamond growth using RF/VHF plasma techniques.
2. The Prior Art
It has been known that synthesis of diamond films can be accomplished using plasma activated chemical vapor deposition processes which now comprise a well-known body of art. Briefly, when energy is applied to a mixture of hydrogen and carbonaceous gases, under certain conditions of temperature, pressure, gas mixture ratio, and other known parameters, excited species are created in the gas excitation zone and are transported by diffusion to suitable substrate surfaces, where they cause films of high-purity polycrystalline diamond to nucleate and grow.
At the time of its discovery, this phenomenon was surprising because it was well known that diamond is not thermodynamically stable under ordinary conditions. The stable form of carbon at normal pressure and temperature is graphite, and the first successful synthesis of diamond was achieved by employing enormous pressure during crystallization under conditions in which diamond is the thermodynamically favored form. The art of low-pressure diamond deposition is now well established and is the basis for a number of commercial applications, including production of diamond-coated cutting tools and diamond microelectronic packages.
Excitation of the gases can take place through a variety of means, including combustion (oxyacetylene flames), heated filaments, plasma arcjets, and microwave plasma excitation. Each of these methods has its limitations. Combustion diamond CVD is not used commercially due to its high cost and difficulty of engineering large-scale deposition systems. Hot filament systems are used to coat tools with diamond, but suffer from low growth rates and from incorporation of filament materials as impurities in the growing diamond film. Plasma torch systems are used to synthesize diamond films, but suffer from high capital and operating costs, and are difficult to scale to deposition sizes greater than a few inches in diameter. Microwave plasma diamond CVD suffers from high capital equipment cost and restricted deposition area, the latter limitation arising from the relationship between plasma excitation zone size and microwave wavelength, in which the plasma zone is commonly limited to approximately 1/4 to 1/2 the wavelength of the energy used to initiate the plasma.
In microwave diamond CVD, microwaves are admitted into a chamber filled with reactant gases. In all published systems, plasma ignition is achieved by causing resonant superposition of at least two microwaves. In many systems, such as those described in U.S. Pat. No. 4,434,188 and those subsequently derived from its teachings, a portion of the microwave energy admitted from one side of the reactor passes through the reaction region and is reflected from an adjustable opposing wall back into the reaction zone. The reflecting wall is adjusted to position the point of highest electric field at the locus of desired plasma ignition, and power is increased until plasma ignition occurs.
Operation of reactors in resonant cavity mode, in which reactor dimensions are at most a few multiples of the wavelength of the excitation energy, is necessitated by the requirements of diamond deposition chemistry. It is known empirically that diamond CVD does not proceed efficiently below operating pressures of approximately 10 Torr, and few if any commercial diamond CVD processes operate below 50 Torr. For example, production of bulk diamond slabs for cutting tools and heat spreaders, often done in plasma torch systems or large microwave CVD systems, takes place at pressures of 100 to 300 Torr. Because of the requirement for high pressure operation, high magnitude electric fields are needed to initiate ionization which is the basis for plasma formation. In consequence, many large-area plasma deposition technologies such as electron cyclotron resonance (ECR) cannot be used for diamond deposition because they do not operate at sufficiently high gas pressures. Resonant cavity operation provides the high electric fields needed to sustain plasmas at the high pressures required for synthesis of high quality diamond.
The need for resonant, standing-wave chamber and microwave technology fixes the physical extent of the plasma excitation region to not more than about one-half the wavelength of the excitation energy, more typically not more than one-quarter thereof. Thus, for the commercially important 915 Mhz microwave frequency, the resultant plasma in diamond CVD systems is about 4 inches in diameter under ordinary operating conditions. Various methods have been developed to circumvent some of the size constraints imposed in deposition of diamond using small microwave plasmas. These include using larger substrates in proximity to the plasma region, extending the plasma through shaping local electric fields, and moving the plasma rapidly across large substrates, as disclosed in U.S. Pat. No. 5,230,740 to Pinneo. These methods provide an ability to diamond coat articles up to about 10 inches in diameter at 915 Mhz with penalties in growth rates and/or deposition uniformity, but the basic difficulty engendered by small plasmas in microwave-driven diamond CVD systems has not been heretofore surmounted.
It would seem that a straightforward remedy for restricted plasma size would be to employ much lower excitation frequencies with longer wavelengths to achieve bigger plasmas. This was the reason for early work in using 13.56 Mhz sources for diamond CVD tests. At 13.56 Mhz, the free space wavelength is in excess of 22 meters, which would give quite large plasmas. In practice, deposition is usually accomplished by applying the RF (radio frequency) energy to an electrode which carries the object to be coated. Inductive coupling, in which a coil external to the chamber is energized with RF, has also been tried for diamond CVD. Neither of these modalities, both well-known to those skilled in the deposition art, has been successful in producing diamond films of usable quality, but rather produce films consisting of mixed forms of carbon, including graphite, amorphous carbon, and small diamond-bonded carbon domains. These films, depending on the properties they exhibit, are often termed "diamond-like carbon" (DLC) films. Their properties are in general inferior to those of diamond; DLC is softer, less conductive of heat, less stable at elevated temperatures, less electrically insulative, etc.
Plasma consists of ionized gases, that is, gases in which electrons have been temporarily removed from their parent molecules by the action of electric fields. Ionization produces the mix of negative, positive, and neutrally charged particles known as a plasma. The charged particles in a plasma are accelerated by electric fields. When alternating electric fields are present (as they are in RF or microwave excited plasmas), charged particles gather kinetic energy during one half of the alternating voltage cycle only to lose it through deceleration during the opposite half-cycle as the field polarity reverses. The more slowly the field reverses (i.e., the lower the driving frequency), the greater the acceleration time, and the greater the kinetic energy imparted to charged particles. At 13.56 MHz (RF), a small portion of the ion population can build up to several hundred electron volts energy.
Since even the strongest chemical bonds have energies less than about 5 eV, ions from RF sources have the energy to break all chemical bonds in materials with which they collide. In contrast, at 2450 MHz (microwave), ion energies are generally less than 10 eV. Microwave plasmas produce ions which have much lower energies and which therefore do not cause disorder when they impact surfaces.
The present inability to produce inexpensive diamond films for industrial use continues to inhibit the development of what otherwise would be a significant industry based on the routine engineering use of CVD) diamond.
It is an object of the present invention to provide a method for CVD deposition of diamond at frequencies below the microwave range.
It is another object of the present invention to provide a method for CVD deposition of diamond which allows depositions over larger areas than prior-art methods.
It is a further object of the present invention to provide a method for CVD deposition of diamond which overcomes some of the problems of the prior art.
It is yet another object of the present invention to provide a method for CVD deposition of diamond which is lower in cost than prior art CVD methods.